This invention is in the field of safety circuits for electrical heating pads and the like which use positive temperature co-efficient (PTC) materials for a heating element. It particularly concerns safety circuits and controls for alternating current heating pads because they usually involve higher voltages.
Heating pads and electric blankets are appliances that, by their nature, conduct high current electrical power in close proximity to the user. Besides the obvious danger of electrifusion as from any electrical appliance, a health concern exists regarding the prolonged exposure to the electromagnetic radiation. Heaters of the PTC type are known to be configured so as to virtually eliminate the magnitude of the electromagnetic fields thought to be harmful. The safe operation of the PTC heating elements is the focus of the present invention.
PTC materials used for heating elements have the added safety of limiting the current draw as the temperature approaches the design limit. With this in mind a heater can be designed without the need for an additional temperature limiting device, such as is disclosed in Crowely U.S. Pat No. 4,271,350. Due to the nonlinear response of temperature with current, sufficient temperature control can be achieved by proportioning power to the heater. The only condition that subverts the inherent safety of the PTC heating element is when the one of the conductors, in intimate contact with the PTC material, breaks and arcing occurs. To prevent this condition from continuing and possibly causing fire, a safety circuit is commonly used that will detect the condition, and then generate a current surge designed to blow the power input fuse, so that the unit is thereby disabled. Carlson, U.S. Pat. No. 4,436,986, teaches the idea of sensing voltage changes and conducting sufficient current to disable the unit when neon bulbs exceed their breakdown voltages. Carlson goes further and incorporates three electrodes within a neon lamp forming a triode that breaks down at a single predetermined voltage, thus reducing the effect of breaks down voltage tolerance. Carlson uses a current limiting resistor to blow the fuse in a predetermined period of time. It is necessary for the current limiting resistor to be rated at a higher power than the fuse to provide a safe open circuit. The fuse, however, must be sized to handle currents of two to three times the continuous current rating of the heater to accommodate the inrush associated with the start up characteristic of the Positive Temperature Coefficient material. The fuse is also relied upon in Carlson's invention to deactivate the unit in all possibilities of short circuits.
A further development that improves the safety of a PTC heating element is taught by Clifford Stine, U.S. Pat. No. 5,081,339. Stine reduces the possibility of breakage and improves the heat dissipation when incorporating a PTC heating wire within a coplanar sandwiched construction, in conjunction with the heating of a waterbed so that the construction is also leak tight. A heat conductive layer and the local current throttling effect of the PTC material combine to provide the most efficient heating without occurrence of hot spots along any part of the heating element.
Typically an adjustable bimetalic control switch is used to provide differing heat settings for the PTC heating. As the current flows through the bimetalic element, it heats up causing the element to bend due to the differential expansion of the metals that comprise the elements. The deflection causes the contacts to open and interrupt the current to the heater and the small bimetalic element to cease bending. The bimetalic element then cools down until contact is again made and the cycle repeats. The deactivation of this type of electro mechanical control, for safety reasons, is best accomplished by blowing a fuse that is in series with the switch.
Modem electrical power controls use solid state switching devices such as Silicon Control Rectifiers, Power Transistors, Solid State Relays and Triacs. Edwin Mills U.S. Pat. No. 4,315,141 uses a pair of solid state switches biased by a temperature sensitive capacitive element as a temperature overload circuit for conventional electric blankets. In these control systems, a small signal controls switching of larger load currents. Integrated circuits or micro processors can be used to provide the control signal required to operate high speed solid state switching. Micro circuits of this type are capable of operating at speeds many times the 50 or 60 HZ. commonly used in AC electrical power supplies. This capability makes it possible to control each AC cycle. In fact, the switching can occur as the AC waveform crosses zero. This type of control can lower the noise generation associated with AC switching and makes the most efficient use of AC power.
Recent advances in microwatt power control has improved the reliability of Integrated Circuits by assuring the proper voltage input to the micro controller. Jamieson and Weiss U.S. Pat. No. 5,196,781 teach an extremely low power voltage detection and switching circuit to provide power input to an IC within a narrow voltage band when only a low power and variable supply is available. Watchdog timing circuits can be incorporated within an IC to perform the task of periodically resetting the IC and to avoid a prolonged lockup or ambiguous operation resulting from power faults and voltage spokes often associated with AC power.